During their normal use and as would be expected, the rail portions of most track structures will be subjected to severe, and uncontrollable environmental conditions. These severe environmental conditions, over a relatively long period of time, may ultimately result in such rail developing certain detrimental flaws. Such flaws may include, for example, cracks.
In addition, in today's modern railroad industry, the rail portion of such track structures will quite often be required to support rather heavy loads being carried by modern freight cars. Furthermore, these heavy loads are travelling at relatively high speeds. It would not be uncommon for these freight cars, when they are fully loaded with cargo, to weigh up to generally about 125 tons. Such relatively heavy loads and high speeds can, also, result in undesirable damage to such rail portions of the track structure. Such damage, for example, may include stress fractures.
It would be expected, therefore, that if these detrimental defects were not timely detected and, likewise, if they are left unrepaired such defects could lead to some rather catastrophic disasters, such as, a train derailment.
As is equally well known, such train derailments are not only costly to the railroad industry from the standpoint of the damage that will likely be incurred to both the cargo being transported and to the railway equipment itself, but, even more importantly, such train derailments may also involve some rather serious injuries, or even worse death, to railway personnel and/or other persons who may be in the vicinity of a train derailment.
It is further well known that a relatively large number of these train derailments have resulted in the undesirable and often costly evacuation of nearby homes and businesses. Such evacuation may be required, for example, when the cargo being transported involves certain highly hazardous chemical products. These hazardous chemical products will generally include both certain types of liquids, such as corrosive acids, and certain types of toxic gases, such as chlorine.
To detect such flaws and defects, ultrasonic testing has been employed. Vehicles have been built which travel along the track and continuously perform ultrasonic testing of the track. These vehicles carry test units which apply ultrasonic signals to the rails, receive ultrasonic signals back from the rails, and provide indications of flaws and defects.
Some of these test units employ sleds which slide along the rails. Acoustic transducers are located in the sleds. These transducers apply ultrasonic signals to the rails, and receive ultrasonic signals back from the rails.
Water is applied to the rails ahead of the sleds to serve as an acoustic bridge between the sleds and the rails. This approach has the disadvantage that it has not been possible to obtain good, constant acoustic contact between the sleds and the rails in heavily curve worn rail. Also such sleds require large amounts of water for adequate sled to rail coupling.
Another approach is to employ small, thin-walled tires which roll along the rails. They are pressed down against the rail so as to have a flat area in contact with the rail. These tires contain acoustic transducers and are filled with a liquid, usually a water-antifreeze solution. The transducers are arranged in an arc to produce acoustic beams which travel through the liquid and are directed toward the center of the flat area. The high frequency electrical transducers are pulsed with energy and the beams intersect in the flat area. The beams then pass through the material of the tire into the rail, are reflected from defects in the rail, the reflected beams returning to the transducers and being detected.
This approach has one disadvantage that only a few transducers can be located in the arc due to spatial considerations. Also, the angles of the acoustic beams produced by the transducers are dictated by their positions in the arc. Another disadvantage is that when the beams strike the inside surface of the tire at the center of the flat area, they generate reflections which cause echoes which reverberate for some time. This limits the times during which signals obtained back from the transducers can be used as indications of beams reflected back from defects and imperfections in the rail.
An additional disadvantage is that for some observations, precise control of the angle of the acoustic beams in the rail is required. This is particularly true for beams which travel along the rail at shallow angles. In this approach, since the beams are generated in liquid, and then, after passing through the tire material, continue in the steel, the beams are refracted by the differing indices of refraction of the acoustic beam between the liquid and steel. Hence, the angle of refraction is affected by the speed of sound in the liquid. For most compositions, this speed depends on the temperature, so the angle of the refracted beam depends on the temperature. To prevent this, it is necessary to use a weak anti-freeze solution for which the speed of sound is temperature invariant. With this weak solution, it is not possible to operate in cold weather.
U.S. Pat. No. 6,055,862 discloses a method of and an apparatus for detecting, identifying and recording the location of defects in a railway rail, which is incorporated herein by reference. The '862 patent discloses a method and apparatus for ultrasonic inspection of an object. A tire is mounted for rolling along the surface of the object. It contains a liquid and a transducer assembly. The transducer assembly is positioned in the tire adjacent a portion of the surface of the tire in contact with the object. The liquid and the surface of the tire in contact with the object provide an acoustic bridge between the transducer assembly and the object. An electric signal processing system is connected to the transducer assembly. It generates electric signals which are converted to acoustic signals in the transducer assembly to place acoustic signals in the object. Acoustic signals returned from the object are converted to electric signals by the transducer assembly, and are processed in the electric signal processing system to indicate flaws in the object.
A problem with some of these prior art systems is that the placement of the transducers may be adversely affected by the defects in the rail which the system is trying to detect. For example, shelling along the gauge side of the rail head may mask defects in the rail. Additionally, gauge face wear may further reduce the effectiveness of transducers located near the typical running surface.